The GC-DFB-LD has various excellent features such as a satisfactory single longitudinal mode property and a resistance to return light inductive noise.
In order to effectuate gain coupling, mainly two methods have been proposed and their characteristics have been reported. As disclosed in a plurality of reports including Japanese Patent Laid-Open Publication No. SHO 60-102788 (first prior art example), a first method is to periodically change the optical gain in an active layer by periodically arranging the active layer itself of a semiconductor laser or providing the active layer itself with a periodic structure (gain diffraction grating). Moreover, as disclosed in a plurality of reports including Japanese Patent Publication No. HEI 6-7624 (second prior art example), a second method is to periodically change a mode gain by periodically arranging an optical absorption layer (absorptive diffraction grating) in the vicinity of the active layer of a semiconductor laser.
The semiconductor laser structures disclosed in the first prior art example and the second prior art example, which are the basic structures for periodically changing the gain, also have a refractive index periodically changed with the gain. That is, in the aforementioned prior art examples, gain coupling and refractive index coupling are intermixed. Therefore, the structures cannot make the best use of the excellent original performance of the gain coupling.
Further, a plurality of reports including Japanese Patent Laid-Open Publication No. HEI 5-136527 (third prior art example) discloses a structure, in which the periodic change of refractive index is canceled in the gain diffraction grating represented by the first prior art example. The structure of the essential part of the GC-DFB-LD disclosed in the third prior art is as shown in a longitudinal cross section in FIG. 19.
In FIG. 19, the material and the layer thickness of each layer are as follows.                Lower clad layer 1: n-type InP; 0.45 μm        Semiconductor layer 2: n-type InGaAsP; 0.2 μm        Buffer layer 3: n-type InP; 10 nm        Active layer 4: i (intrinsic)-InGaAsP; 0.1 μm        Guide layer 5: p-type InP; 1.2 μm        Upper clad layer 6: p-type InP; 1.2 μm        
This structure is obtained by laminating the lower clad layer 1 and the semiconductor layer 2 on an InP substrate through first-time crystal growth, thereafter forming a diffraction grating shaped corrugated configuration 7 on the surface of the semiconductor layer 2 by a two-beam interference exposure method and an etching technique and laminating the layers of the buffer layer 3 up to the upper clad layer 6 on the semiconductor layer 2 through second-time crystal growth.
In this case, the active layer 4 has a periodic structure under the influence of the corrugated configuration 7 of the semiconductor layer 2 of its groundwork, and this modulates the gain, causing gain coupling. On the other hand, refractive index distribution is increased in order of the guide layer 5, the semiconductor layer 2 and the active layer 4 by material selection. As a result, in a region A–A′ in FIG. 19, the active layer 4 of a large refractive index has a large volume, and the guide layer 5 of a small refractive index also has a large volume. Therefore, the large refractive index of the active layer 4 is canceled. On the other hand, in a region B–B′, the active layer 4 of a large refractive index has a small volume, and the guide layer 5 of a small refractive index accordingly has a small volume. Thus, by controlling the corrugated configuration 7 of the semiconductor layer 2, the post-burial configurations of the active layer 4 and the guide layer 5, which bury it, and the refractive indexes of the layers, it is enabled to achieve a balance so that an equivalent refractive index becomes constant not only in the regions A–A′ and B–B′ but also in arbitrary regions. Thus, a GC-DFB-LD, which substantially has no refractive index coupling, can be obtained.
Furthermore, a plurality of reports including Japanese Patent Publication No. HEI 8-8394 (fourth prior art example) and Japanese Patent Laid-Open Publication No. HEI 5-29705 (fifth prior art example) disclose structures for canceling the periodic change of refractive index in the absorptive layer in an absorptive diffraction grating represented by the second prior art example.
In the aforementioned third through fifth prior art examples, a refractive index perturbation caused by the provision of the corrugated configuration in the active layer or the absorptive layer is canceled by providing an anti-phase refractive index perturbation in the neighborhood. The GC-DFB-LD, which substantially contains no refractive index coupling component, is called the intrinsic GC-DFB-LD.
However, the aforementioned third through fifth prior art examples have the following problems. That is, in the aforementioned third through fifth prior art examples, the refractive index distribution is canceled by the well-balanced provision of the anti-phase refractive index distribution for the active layer or the absorptive layer having the corrugated configuration. This theoretically requires an extremely high processing accuracy in controlling the corrugated configurations of the gain diffraction grating and the absorptive diffraction grating as well as the burial configuration of the buried layer. That is, the perturbation of the equivalent refractive index disadvantageously largely changes even with a little change in the diffraction grating and burial configurations, and this disadvantageously puts the refractive index distribution canceling balance into disorder.
Moreover, a plurality of devices are normally collectively fabricated in a wafer. In the above case, it is difficult to avoid the influences of the variations in shape of the diffraction gratings in the wafer and the variations in shape occurring every production lot, and a thorough process management is indispensable in order to obtain the intrinsic GC-DFB-LD.